Unlock Alternate Costumes:
At the 'Character Selection' screen highlight a fighter then press Up or Down.

Unlock Gargos:
At the 'Character Selection' screen hold Up and press Fierce Punch, Medium Punch, Medium Kick, Fierce Kick, Medium Punch, Quick Punch, Quick Kick, Medium Kick.

Unlock Faster Game:
At the 'Character Selection' screen choose 2-Player mode and then press Down + Medium Punch or Medium Kick before selecting a fighter.

Reset Speed to Normal:
At the 'Character Selection' screen choose 2-Player mode and then press Down + Quick Punch or Quick Kick before selecting a fighter.

Stage and Music Select:
At the 'Character Selection' screen choose 2-Player mode and enter one of the following codes while picking a fighter to unlock the corresponding effect (the first player to enter a code will select the stage and the second player to enter a code will select the music).

Unlock Bridge:
Press Down + Quick Punch.

Unlock Museum:
Press Up + Medium Kick.

Unlock Jungle:
Press Up + Medium Punch.

Unlock Spaceship:
Press Up + Fierce Punch.

Unlock Helipad:
Press Up + Fierce Kick.

Unlock Stonehenge:
Press Up + Quick Kick.

Unlock Sabrewulf's Castle:
Press Up + Quick Punch.

Unlock Ship:
Press Down + Fierce Kick.

Unlock Spinal's Ship:
Press Down + Medium Kick.

Unlock Dojo:
Press Down + Quick Kick.

Unlock Street:
Press Down + Fierce Punch.

Unlock Dungeon:
Press Down + Medium Punch.

Unlock Sky:
On both sets of controls press Down + Medium Kick simultaneously.